Carbon nanotubes are cylindrically rolled graphite sheets, at both ends of which there exists a half fluorene. These allotropes of graphite were first produced by an electric arc discharge method, which process finally leads to multi wall carbon nanotubes (MWCNTs) as a main product. Later on, chemical vapor deposition techniques were used and single wall carbon nanotubes (SWCNTs) were deposited on catalysts, using hydrocarbon vapor as the feed.
CNTS have unique properties, which makes them useful for different purposes, among which is their ability to be used as catalyst support material. Also, they are useful in fuel cells, for gas storage, within electronic devices, for example electronic microscopes can be highlighted. Depending on the application area, the type and size of CNTS should be controlled during the process for producing the same.
In the art, there are three major methods described for the production of CNTS:
These methods include using an electric arc on a graphite target (Journal C., et al, Nature, 388, 756, 1997), laser ablation (Thess A. et al, Science 273, 483ff, 1996) and catalytic chemical vapor deposition of hydrocarbons (Li A. et al, Science, 274, 1701ff, 1996).
MWCNTs are presently produced in an industrial scale, however, single and double wall CNTS can only be produced in a scale of some hundred grams, when making us of the presently available facilities described in the art.
Dai et al. describes in Chemical. Physics. Lett. 260, 471 (1996) a selective synthesis of single wall carbon nanotubes, (SWCNTs) through decomposition of hydrocarbons over Mo/Al2O3, using VIII and VI group metals like Co and Mo, on other supports like SiO2.
According to Resasco et al. single wall carbon nanotubes are produced and they also disclose a continuous process for CNT production in US 2005/0042162. A drawback of this process described in the art, however is, that it cannot be run in a final continuous manner since the catalyst has to be produced in an independent step, which comprises several preparation and calcinations steps. According to this prior art method, single wall carbon nanotubes (SWCNTS) can be produced in a fluidized bed reactor, which—due to the inability of particles to fluidize—makes necessary a “prior-to-use catalyst shaping step”, for limitation of the particle site to a range suited for fluidizing. That increases the residence time in the reactor and the CNTs produced by the said process must be purified by several steps after the production.
Fei, Wei et al. introduced a method for continuous production of CNTs (US 2004/0151654) according to which the catalyst is separately prepared by impregnation and then CNTs are grown over the said catalyst in a fluidized bed reactor. Fe—Cu, Ni—Cu, Co—Mn, Ni catalysts having supports based on SiO2, Al2O3 and glass beads were used in the said process. The application of a fluidized bed reactor, however, leads to before mentioned drawbacks ending up with poor quality of the produced CNTS.
Generally speaking, the yield of CNTs during their production is affected by several factors, such as catalyst formulation, operation parameters (including reaction temperature, pressure, residence time and oxidation/reduction and calcinations conditions), and also the type and design of the used apparatus.
Presently, the CVD (chemical vapor deposition) method is still a common technique for mass production of CNTs although it suffers from problems of time-dependent low yield, high investment and operational costs that are caused due to the high number of process steps being necessary after the production of the catalyst as well as the CNTs and still further due to the problem that the product is distributed throughout the reactor.